In order to enable a user to read the information carried by a dial when the ambient light is weak, or even in the dark, designers have conceived numerous solutions, of which only those that use an electrical, micro-bulb, diode or other light source will be mentioned within the scope of the present invention.
A single diode arranged in the housing of a flange, between the crystal and the dial, evidently does not provide uniform lighting and adding more diodes around the dial still leaves areas of shadow. This can be satisfactory when the desired objective is essentially technical, for example an automobile vehicle dashboard, but this is not the case when there is also an aesthetic concern, as is the case in a timepiece.
In order to have more uniform lighting, numerous Patents propose injecting light from the light source into a wave-guide arranged around the dial on the flange, or replacing the latter. U.S. Pat. Nos. 5,984,485 and 6,452,872 disclose for example guides whose surfaces have particular structuring for reducing the influence of losses along the guide and for reorienting the light towards the dial. The lighting is more uniform than before, but not entirely satisfactory. Moreover structuring the guide is a difficult operation, both from the production point of view and from the reproducibility point of view, and contributes eventually to increasing the cost.
In order to have uniform lighting, certain documents disclose devices wherein the light source is placed at the centre of the dial, on the crystal, and directly lights the subjacent dial and the markings that it bears.
Such a device, for example disclosed in U.S. Pat. No. 4,115,994, comprises a light source positioned on the lower face of a crystal of unequal thickness and connected to the energy source by two juxtaposed conductive wires, embedded in the thickness of the crystal. U.S. Pat. No. 4,118,924 discloses a device wherein at least one light source is also positioned on the lower face of a crystal of equal thickness while being connected by two conductive wires in the extension of each other. In this document, one embodiment discloses lighting by means of a plurality of diodes arranged above the grids of a digital display, the conductive wires of each diode in a way creating a “spider web” in the crystal. Such direct lighting allows annular distribution of the light, but does not provide uniform lighting of the whole dial. Moreover, the presence of conductive wires in the crystal produces an unattractive effect unsuitable for a timepiece.
U.S. Pat. No. 6,106,127 proposes reducing the aforementioned drawback by passing the conductive wires through a hole arranged through the arbour of the hands, which is a step towards a more attractive embodiment, but does not remove the problem of halo effect lighting.
The hand arbour was also used for example in the past as a light guide. U.S. Pat. No. 3,859,782 discloses for example a device wherein a light source is arranged at the back of the dial facing the hour wheel pipe, the other end of which opens out opposite a small reflector bonded under a crystal whose two faces are parallel. The drawback regarding the wires does not exist, but, in addition to the difficulty of passing the wires through the hour wheel pipe without subsequently adversely affecting the free rotation thereof, “uniform” lighting of the dial cannot be obtained with a reflector returning the incident light directly onto the dial.